There are many applications where it is desirable to test or measure the internal pressure within a flexible hydraulic or pneumatic line. This may be for safety purposes, or for other reasons such as accuracy of measurement of flow conditions.
In both underground and open-cut mining applications hydraulic lines are commonly used on a variety of equipment ranging from jacking and roof bolting equipment to excavating and long wall shearing devices. As such, hydraulic motors, pumps and pistons provide an integral part of most mining environments. In a similar way, hydraulic equipment is widely used by other industries such as forestry, farming and construction.
At certain stages of operation of hydraulic machinery, it may become necessary to depressurise and decouple a hydraulic line. During such a process it is important that the hydraulic line is firstly depressurised, to reduce the risk of any personnel being injured by the high pressure hydraulic fluid. Despite safety protocols, accidents still occur. The residual pressure in the hydraulic line may still be too high in some instances, even after depressurisation has been attempted. Alternatively, technicians often incorrectly assume that a line has a low pressure or no pressure.
Previously, hydraulic couplings utilised corresponding male and female threaded members. The threaded couplings meant that when a user believed the hydraulic line to have been depressurised, by slowly unscrewing the thread, the hydraulic fluid would leak immediately after the seal was broken, indicating to the user if the line still contained high pressure fluid.
However; in newer machinery, hydraulic lines are typically coupled together with snap lock type fittings. Whilst snap lock fittings are fast to connect and disconnect, they also provide the disadvantage of being very dangerous if they are disconnected while the line pressure is still too high. Because the snap lock fitting is fast to disconnect, there are incidents of personnel being injured and killed by the hose whipping around, and either striking a person, or spraying the person with hydraulic fluid which may be at very high pressures, and temperatures.
There are known invasive devices for testing the pressure in a hydraulic line. These devices include deadweight pressure testers, pressure transducers and hydraulic multi-meters. However, these devices are all invasive and as such require connection to the hydraulic line through valves, ports and other means. A disadvantage with these devices is that they are not portable and only provide a pressure reading at a given location in the hydraulic circuit. A further disadvantage is that it is not always possible to connect an invasive testing device to a hydraulic circuit. For example, in a complex hydraulic circuit with long lines, not all lines are provided with a test point to allow pressure measurement.
There are known non-invasive devices for testing fluid flow at a specific point in a hydraulic line. Examples of such devices include strain gauges which require prior installation into the line and calibration. A disadvantage of using such devices is that they often do not provide a sufficient degree of precision, and they tend to be expensive and complex to operate.
Other fluid pressure testing devices include metal discs moving within the electromagnetic field of an energised coil to sense pressure changes. Such devices generally use a test chamber separated into two parts by a flexible diaphragm. However, these devices are intended for medical applications and other such uses where the fluid pressure in the line is relatively small.